1. Field of the Invention
This invention relates to a method of controlling the speed of rotation of a motor. More specifically, to a method of controlling the speed of rotation of a motor such that the value of a resistor circuit connected in series to the motor and having discontinuous resistance values is changed to control the speed of rotation of the motor to a substantially fixed level irrespective of a variation of a load to the motor. Above all, the present invention relates to a controlling method which can be applied to a device for controlling the speed of rotation of a starter motor for a vehicle which makes use of a driving force of the starter motor upon backward running of the vehicle.
2. Description of Background Art
A backward running device for a vehicle such as an autobicycle which makes use of driving force of a starter motor upon backward running of the vehicle is already known. One example is set forth in Japanese Patent Laid-Open No. 62-157882. A controlling mechanism of a simple structure for such a device includes current flowing through a starter motor which is changed by changing over a plurality of resistors provided between the motor and a power source to change the output power of the starter motor to cause the vehicle to smoothly run backwardly at a reduced speed.
For example, three resistors having different resistance values may be provided between a starter motor and a power source. If the resistors are changed over in response to a speed of rotation of the starter motor, then the current flowing through the starter motor and the speed of rotation of the motor have a relationship as shown in FIG. 5. In particular, the relationship between the motor current and the motor rotational speed changes along a curve, identified as curve A, when the motor current flows by way of a resistor having a high resistance value. However, the relationship changes along a curve, identified as curve B, when the motor current flows by way of another resistor having a medium resistance value. In addition, when the motor current flows by way of the remaining resistor having a low resistance value, the relationship changes along a curve, identified as curve C.
Accordingly, since there is a proportional relationship between the motor torque necessitated to cause the vehicle to run backwardly, that is, the motor load, and the motor current, it can be anticipated that the motor rotational speed is kept substantially constant irrespective of a change of the motor load by causing, while the motor load is low, the motor current to flow by way of the resistor of the high resistance value and by changing over the resistor to another resistor having a lower resistance value as the motor load increases.
However, while the changing over of the resistors is performed in response to a motor rotational speed, according to the setting of a motor rotational speed at which such changing over of the resistors is to be performed, the actual driving torque of the motor does not coincide with a torque required for the motor. It is possible that the speed of rotation of the motor may widely vary. In particular, it is assumed that, in the case of the curve A of FIG. 5, when the motor rotational speed is decreased, for example, because the required torque is high, and it is changed over, for example, at a point Al of the curve A at which the speed of rotation is 800 rpm, to a point B1 of the curve B. However, if such a driving torque as can be obtained at the point B1 is not required, then the motor current decreases and the motor rotational speed increases. Then, if the driving torque is higher than a required torque even after, for example, a point B2 of the curve B at which the speed of rotation is 1,100 rpm is reached, that is, when the amount of the motor current corresponding to the required torque is in a region D between an amount of current at the point A1 and another amount of current at the point B2, the motor rotational speed again returns from the point B2 of the curve B to point A2 of curve A. Accordingly, when the required torque remains in a torque amount section such as represented by a current amount section D, the required torque does not coincide with a driving torque obtained in accordance with curve A nor with a driving torque obtained in accordance with curve B. Thus, changing over of control of the resistors falls into an endless loop which passes the points of A1, B1, B2 and A2. Consequently, a problem takes place wherein the speed of rotation of the motor increases and decreases frequently between 800 rpm and 1,100 rpm and is not stabilized.